I. Field
The following description relates generally to wireless communications, and more particularly to reducing a rank associated with a user device as a number of transmissions increase within a transmission frame or frames in a wireless communication environment.
II. Background
Wireless communication systems have become a prevalent means by which a majority of people worldwide has come to communicate. Wireless communication devices have become smaller and more powerful in order to meet consumer needs and to improve portability and convenience. The increase in processing power in mobile devices such as cellular telephones has lead to an increase in demands on wireless network transmission systems. Such systems typically are not as easily updated as the cellular devices that communicate there over. As mobile device capabilities expand, it can be difficult to maintain an older wireless network system in a manner that facilitates fully exploiting new and improved wireless device capabilities.
More particularly, frequency division based techniques typically separate the spectrum into distinct channels by splitting it into uniform chunks of bandwidth, for example, division of the frequency band allocated for wireless communication can be split into 30 channels, each of which can carry a voice conversation or, with digital service, carry digital data. Each channel can be assigned to only one user at a time. One known variant is an orthogonal frequency division technique that effectively partitions the overall system bandwidth into multiple orthogonal subbands. These subbands are also referred to as tones, carriers, subcarriers, bins, and/or frequency channels. Each subband is associated with a subcarrier that can be modulated with data. With time division based techniques, a band is split time-wise into sequential time slices or time slots. Each user of a channel is provided with a time slice for transmitting and receiving information in a round-robin manner. For example, at any given time t, a user is provided access to the channel for a short burst. Then, access switches to another user who is provided with a short burst of time for transmitting and receiving information. The cycle of “taking turns” continues, and eventually each user is provided with multiple transmission and reception bursts.
Code division based techniques typically transmit data over a number of frequencies available at any time in a range. In general, data is digitized and spread over available bandwidth, wherein multiple users can be overlaid on the channel and respective users can be assigned a unique sequence code. Users can transmit in the same wide-band chunk of spectrum, wherein each user's signal is spread over the entire bandwidth by its respective unique spreading code. This technique can provide for sharing, wherein one or more users can concurrently transmit and receive. Such sharing can be achieved through spread spectrum digital modulation, wherein a user's stream of bits is encoded and spread across a very wide channel in a pseudo-random fashion. The receiver is designed to recognize the associated unique sequence code and undo the randomization in order to collect the bits for a particular user in a coherent manner.
A typical wireless communication network (e.g., employing frequency, time, and code division techniques) includes one or more base stations that provide a coverage area and one or more mobile (e.g., wireless) terminals that can transmit and receive data within the coverage area. A typical base station can simultaneously transmit multiple data streams for broadcast, multicast, and/or unicast services, wherein a data stream is a stream of data that can be of independent reception interest to a mobile terminal. A mobile terminal within the coverage area of that base station can be interested in receiving one, more than one or all the data streams carried by the composite stream. Likewise, a mobile terminal can transmit data to the base station or another mobile terminal. Such communication between base station and mobile terminal or between mobile terminals can be degraded due to channel variations and/or interference power variations.
Conventional MIMO wireless systems utilize a “static MIMO transmission rank” for multiple transmissions of a packet or multiple packets. There are multiple scenarios where such a transmission limitation can lead to performance degradation.
Reverse Link Control Overhead Limitation: Typical MIMO systems employ a feedback channel from the Access Terminal to the Access point, where a CQI and desired MIMO transmission rank information is sent every few milliseconds. In heavily loaded (high traffic with lots of users) scenario, there may be a delay in this CQI & rank feedback due to limited reverse link feedback capacity or high reverse link channel erasures. In the absence of updated CQI & rank information, Conventional MIMO systems that are unable to update the MIMO transmission rank over multiple transmissions of a packet or multiple packets seemlessly, may not be robust to changing channel conditions, interference levels, leading to late HARQ transmission decodes or packet decode failures.
Forward Link Control Overhead Limitation: To enable successful packet decode, the Access Point needs to signal the Rank and Packet format to the receiver, using the shared control channel in the FL. However, in a heavily loaded scenario, the Access Point may not be able to signal the Rank and Packet format to the receiver, for every transmitted packet, due to overhead limitations. As a result, Conventional MIMO systems that are unable to update the MIMO transmission rank over multiple transmissions of a packet or multiple packets seamlessly, may not be robust to changing channel conditions, interference levels, leading to late HARQ transmission decodes or packet decode failures.
Partial Loaded Scenario: In partially loaded scenario (few users in the network with bursty traffic), the interference levels over multiple HARQ transmissions of a frame can be substantially different. Furthermore, the CQI and rank information computed at the Access Terminal are typically computed using pilots that suffer from a different interference level, relative to data transmission. A MIMO transmission scheme that utilize a “static MIMO transmission rank” for multiple HARQ transmissions of a packet, may not be robust to such interference variations, leading to late terminations and packet decode failures.
Code Repetitions: In systems that employ HARQ, the code-rate decreases with increasing HARQ transmissions. This is because more redundant symbols are sent with increasing HARQ transmissions. Code rates below the base code-rate (say rate 1/5) are achieved via symbol repetitions. Unfortunately, symbol repetitions lead to performance loss. One way to avoid symbol repetitions is to transmit fewer redundant bits over later transmissions to maintain the effective code rate greater than the base code-rate. MIMO transmission schemes can (potentially) easily achieve this by lowering the rank of the MIMO transmission over later HARQ transmissions. Unfortunately, MIMO systems that utilize a “static MIMO transmission rank” cannot accomplish the rank change. As a result, the performance of MIMO systems that utilize a “static MIMO transmission rank” suffers with increasing HARQ transmission.
Due to many reasons mentioned above, there exists a need in the art for a system and/or methodology of improving throughput and reliability in MIMO wireless network systems, by seamlessly varying the rank of the MIMO transmission across multiple HARQ frames, and across multiple packets.